Field of the Invention
The present invention relates to combined MRI and radiation therapy equipment.
Description of the Prior Art
Certain examples of combined MRI and radiation therapy equipment are known, but suffer from certain drawbacks. The present invention addresses at least some of those drawbacks.
Radiation therapy typically uses gamma radiation, or similar, to target cancerous tissues in a patient. Such radiation may be generated either using an electron beam generated by an accelerator and aimed at a suitable target, or a radiation source containing a suitable radioactive isotope, such as cobalt-60.
The use of a cobalt-60 source involves difficulties in storage, and prevention of over-exposure by operators. Screening of such sources can only be achieved by significant amounts of dense material such as lead or tungsten. Such sources are simple and are not affected by magnetic fields such as would be encountered in a combined MRI and radiation therapy system. The radiation produced is however of relatively low energy, and cannot be intensity-modulated.
Radiation generation by electron beam acceleration onto a suitable target has the advantages of being able to produce higher-energy photons, and may be intensity modulated.
The accelerators required to produce an electron beam, typically linear accelerators, are very sensitive to transverse magnetic fields, which makes their incorporation into MRI equipment difficult. The magnetic field deflects the path of the electron beam within the accelerators, destroying the efficacy of such radiation sources.
An example arrangement which allows a linear electron accelerator (LINAC) to be built into an MRI system and used for combined MRI and radiation therapy is known from WO2003008986 and uses a radially-aligned LINAC. The LINAC and its associated target are arranged to project a radiation beam through an aperture or transparent window in a cryostat, between coils of a superconducting MRI magnet. The radially-aligned LINAC is arranged on the mid-plane of the magnet, and requires a lot of space around the magnet, making it impractical for many installations. The magnetic field of the main magnet is transverse to the LINAC, and interferes with the electron beam path. Only relatively low main magnet field strength (flux density) can be tolerated.
A more compact arrangement of combined MRI and radiation therapy equipment is described in US Patent publication US2011/0213239A1, International Patent publication WO2012049466 and UK Patent GB2484529. In this arrangement, the linear accelerator (LINAC) is arranged parallel to the axis of the magnet and is situated between gradient coils and the main magnet field coils of an MRI system. Beam steering arrangements are provided to deflect the generated electron beam from an axial path, parallel to the axis of the magnet, to a radial path, perpendicular to the axis, and then onto a suitable target. The LINAC and target are accordingly immersed in a relatively strong magnetic field.
FIG. 1 corresponds to FIG. 1 of WO2012049466, US2011/0213239A1 and GB2484529. It shows a schematic representation of a conventional combined radiation therapy and magnetic resonance unit 1 with a magnetic resonance imaging part 3 and a radiation therapy part 5. The magnetic resonance imaging part 3 includes a main magnet 10, a gradient coil system having two (in this case symmetrical) partial gradient coils 21A, 21B, radio-frequency coils 14, for example two parts of a body coil 14A, 14B, and a patient bed 6. All these components of the magnetic resonance imaging part 3 are connected to a control unit 31 and an operating and display console 32.
Both the main magnet 10 and the partial gradient coils 21A, 21B are essentially shaped like a hollow cylinder and arranged coaxially around the horizontal axis 15. The inner shell of the main magnet 10 limits in radial direction (perpendicular to the axis 15) a cylinder-shaped interior 7, in which the radiation therapy part 5, the gradient system, high-frequency coils 14 and the patient bed 6 are arranged. More precisely the radiation therapy part 5 is located in the interior 7 between a radially outer side of the gradient coil system 21A and 21B and a radially inwardly facing surface of a housing of the main magnet 10.
In addition to the magnet coils, the main magnet 10 comprises further structural elements, such as supports, housing etc., and generates a homogenous main magnetic field necessary for magnetic resonance imaging. In the example shown, the direction of the main magnetic field is parallel to the horizontal axis 15. High-frequency coils 14 are used to excite nuclear spins in the patient. The signals emitted by the excited nuclear spins are received by the high-frequency coils 14.
The axially spaced-apart partial gradient coils 21A, 21B in each case include gradient coils 20, which are in each case completely enclosed by a shield 27. The gradient coil 20 has supports and individual gradient coils that generate magnetic gradient fields for selective layer excitation and for location-coding of the magnetic resonance signals in three spatial directions.
The radiation therapy part 5 is arranged on a gantry 8 and comprises a linear electron accelerator (LINAC) 9, a beam deflection arrangement 17, a target anode 19, a homogenizing body 22 and a collimator 23. The gantry 8 can feature a through-hole (broken lines), by which access to the magnetic resonance imaging 3 part is possible, through the gantry.
The LINAC 9 has an electron source 11, for example a tungsten cathode, which generates an electron beam 13, which is accelerated parallel to the axis 15 of the main magnet 10. If the LINAC 9 generates pulsed electron beams 13, it can be built more compactly than one designed to provide a continuous electron beam. The LINAC 9 for example may generate electron beam pulses with a length of 5 μs every 5 ms.
The electrons of the electron beam 13 are accelerated by electric alternating fields in cylinder-shaped hollow conductors of the LINAC 9. The electrons of the electron beam 13 are accelerated to energies up to a magnitude of several MeV. The LINAC 9 is connected to an accelerator control unit 12 to control the alternating fields and the electron source 11.
The electron beam 13 leaves the LINAC 9 at the end opposite the electron source and is deflected by the beam deflection arrangement 17 through 90° radially inward towards axis 15. For this purpose the beam deflection arrangement 17 may have a magnet configured as an electromagnet made of non-ferromagnetic materials to prevent undesired interaction with the surrounding magnetic fields.
To be able to deflect the pulsed electron beam 13 in a small space, the beam deflection arrangement 17 must generate strong magnetic fields. To reduce the power loss, the magnetic field of the beam deflection arrangement 17 is a pulsed electro-magnetic field which is synchronized with the pulsed electron beam 13. For this purpose the beam deflection arrangement 17 is connected to a beam deflection control unit 18 which is also connected to the accelerator control unit 12.
The deflected electron beam 13 hits the target anode 19 and generates a radiation beam that emerges from the target anode in the beam elongation along a beam path. The radiation beam is homogenized by the homogenizing body 22.
The collimator 23 is arranged in an annular slot between the distanced partial gradient coils 21A, 21B in the beam path after the target anode 19. The proximity to the irradiation target thus achieved improves the radiation luminance and the effectiveness of the collimator 23.
The collimator 23 enables the direction of the radiation beam and the cross-section of the radiation beam to be influenced. For this purpose the collimator 23 preferably incorporates moveable adjusters 24, which permit the radiation beam to pass only in a certain direction, e.g. only parallel to the radial direction 26 or up to an angle α away from the beam axis 26, and with a certain cross-section. It is also possible to set the adjusters 24 of the collimator 23 in such a way that no radiation beams can pass parallel to the radial beam axis direction 26 and only angled radiation beams at certain angles from the radial direction 26 can pass through. To control the adjusters 24, the collimator 23 is connected to a collimator control unit 25. Such collimators are adequately known. By way of example, reference can be made to multi-leaf collimators. They make it possible to perform intensity modulated radiation therapy (IMRT), in which the size, shape and intensity of the radiation beam can be optimally adapted to the irradiation target. In particular IMRT also enables the irradiation center to be positioned outside the rotational axis of the radiation therapy device.
The radiation beam penetrates the examination subject, in this case the patient P, and the radiation beam path runs through a diagnosis (imaging) volume D of the magnetic resonance imaging part 3. To minimize the local dose of radiation outside the irradiation target volume, the radiation therapy part rotates around the axis 15 of the main magnetic field. As a result, the full dose is applied only in the irradiation center B. The collimator 23 constantly adapts the cross-section of the radiation beam to the actual outline of the irradiation target even during rotation. The gantry 8 is configured for rotation of the radiation therapy part. A gantry control unit 29 controls the movement of the radiation therapy part 5. As an example the radiation therapy part 5 is shown as radiation therapy part 5′ after rotation through 180°.
The gantry control unit 29, the collimator control unit 25, the beam deflection control unit 18, the accelerator control unit 12 and the control unit 31 are connected to each other so that the diagnosis data collected by the magnetic resonance imaging part, for example the three-dimensional shape of the irradiation target, the rotational position of the radiation therapy part, as well as the collimator settings with regard to cross-section and direction of the radiation beam and the generation of pulsed beams described above can be coordinated with each other.
The patient bed 6 is preferably moveable in three spatial directions so that the target area of the irradiation can be positioned precisely in the irradiation center B. For this purpose the control unit 31 is expediently configured for controlling a movement of the patient bed.
This known arrangement, however, suffers from certain disadvantages. By locating the LINAC 9 and the target 19 within the main magnet 10, the coils of the main magnet must be of relatively large diameter, and the LINAC and target must be located close to the main magnet coils, in order to keep the overall size of the system to an acceptable diameter. Operation of this arrangement has been demonstrated experimentally, but only where the magnetic field experienced by the LINAC 9 is of sufficient homogeneity. This is difficult to achieve when the LINAC is positioned close to the main magnet coils, as the electron beam quality may be degraded by variations in the magnetic field experienced by the electron beam due to variations in magnetic field orientation and strength. The magnetic resonance imaging part is designed to generate a homogeneous magnetic field in a central imaging region, and the magnetic field in the volume occupied by the LINAC 9 is rather less homogeneous. The magnetic field will be strong within the bore of the main magnet 10, but the magnetic field lines in the region will not be truly parallel, particularly near the end of the magnet, and some deflection and dispersion of the beam will result.
The arrangement of FIG. 1 does not allow much space for radiation beam shaping devices such as multi-leaf collimator (MLC) conventionally and advantageously provided in radiation therapy equipment.